The invention relates to a cathode having a layer of electron-emissive material comprising alkaline earth material oxides, which oxides include at least barium oxide, and a rare earth metal, the layer being coated on a supporting body substantially comprising nickel.
The invention also relates to a method of manufacturing such a cathode, and to an electron beam tube provided with such a cathode.
The emission of such cathodes is based on the release of barium from barium oxide. In addition to the barium oxide, the electron-emissive material usually comprises strontium oxide and sometimes calcium oxide.
The actual emission is mainly ensured by small areas (so-called "sites") having the lowest effective electron work function, which sites are spread over the electron-emissive material. In practice, sites having a slightly higher work function will hardly contribute to the electron current generated by the cathode.
For a high effective electron emission it is therefore favourable to choose the number of sites having a minimal work function and the distribution of the sites over the emissive layer as optimally as possible.
In U.S. Pat. No. 4,797,593, improved electron emission properties are obtained by the addition of certain rare earth metals in amounts of at least 0.05% by weight.